This invention relates to a multifunction network adapted to be constructed using very large scale integrated circuit techniques and capable of being controlled to operate in one of four selected modes of operation as a bidirectional bus multiplexer, an error correction assist circuit, a register stack addressing circuit for either direct or indirect addressing, or a multiport file.
In the construction of large scale data processing circuits it has long been the custom to construct the circuits in modular form with each module (chip, card, etc.) being dedicated to performing a particular function or functions. This approach has required a large number of different types of modules. As the techniques for fabricating integrated circuits have improved, larger and larger integrated circuits have been fabricated, and fabrication costs have been reduced. Thus, very large scale integrated circuits (VLSI) can now be fabricated at reasonable expense. However, because of space considerations it is desirable to keep to a minimum the number of external connections to the chip.
Certain data processing functions have enough commonality that, in forming a circuit to perform a first function, very little additional circuitry need be added so that a second function may be performed. The present invention utilizes this fact together with VLSI techniques to provide a chip which requires very few terminal connections compared to its data handling capabilities, and which may be programmed by voltages hardwired to two terminals to operate in any one of four modes. Thus, a multifunction network according to the present invention may be plugged into one receptacle having a first combination of voltages available at the mode selection terminals, and will operate as, for example a bidirectional bus multiplexer. The same multifunction network may alternatively be plugged into a second receptacle having a different combination of voltages at the mode selection terminals, and the multifunction network will operate as an addressing circuit for direct or indirect stack addressing. Since the multifunction network can be selectively programmed for different modes of operation, fewer different types of networks are required in order to build up a large scale modular data processing system. In addition, it offers the advantage that if a portion of a chip performing a first function in a system should fail, it might be interchanged with a chip performing a different function in order to keep the system operating. This of course assumes that the failed portion of the chip is not used in both functions.